making part of package optional at ./configure time

[mick]

I am trying to make the inclusion of a module of my program an option 
configurable in ./configure but can't get it right.

At the moment I have 
in configure.ac
....
AC_ARG_ENABLE(webcam,
  [  --disable-webcam        disable use of webcam])
AC_ARG_ENABLE(voice,
  [  --disable-voice         disable use of voice])
....
AC_DEFINE([USE_WEBCAM], [], [enable/disable webcam broadcaster])
AC_DEFINE([USE_VOICE], [], [enable/disable voice chat])
....

in config.h.in
....
/* enable/disable voice chat */
#undef USE_VOICE

/* enable/disable webcam broadcasterDescription */
#undef USE_WEBCAM
....
and 
#ifdef USE_VOICE
...
#endif
or 
#ifdef USE_WEBCAM
...
#endif

surrounding the relevent code in the source but this doesn't seem to be 
working.

Can someone please pointme towards clear explaination of how to do this AND/OR 
a solution or example

/]/]ik